Nonlinear scan drive reader with variable clock correction

ABSTRACT

A mirror driven by a resonant mechanical oscillator is used to scan a laser beam across a document. The incident laser beam line scans the record and is absorbed or reflected depending upon the presence or absence of an image on the record. The reflected intensity modulated light is detected by photomultiplier tubes which provide a video analog signal. An A/D device digitizes the analog signal forming a DATA OUT signal. The laser beam scan velocity across the document varies sinusoidally because of the resonant nature of the drive system. The data flow, or bit density of DATA OUT, has a corresponding sinusoidal variation. A write rate controller provides a variable clock which accompanies the DATA OUT. The variable clock has a periodic variation which matches the scan velocity. The rate of the variable clock matches the bit density of DATA OUT causing DATA OUT to be entered into a data bank properly notwithstanding its nonuniform bit density.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to readers with nonlinear scanning systems whichproduce a nonlinear bit density output, and more particularly to suchreaders having a complementary nonlinear clock which accompanies theoutput and compensates for the nonlinearity.

2. Description of the Invention

Heretofore, sinusoidal variations in mechanical oscillator scanningrates have been avoided by using a multifacet rotating mirror or prismwith a constant speed drive. The incredible machine tolerance ingrinding and polishing each of the facets creates a huge cost barrier.The constant speed drive requirement is a heavy maintenance burden.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide:

DIGITAL READER WITH A NONLINEAR CLOCK WHICH COMPENSATES FOR THENONLINEAR BIT DENSITY OF THE READER OUTPUT;

THE ABOVE READER EMPLOYING A NONLINEAR SCAN LINE DRIVE MOTION;

THE ABOVE READER EMPLOYING A SINUSOIDAL OSCILLATOR WITH A STABLE NATURALFREQUENCY OF RESONANCE;

THE ABOVE READER EMPLOYING A SIMPLE MAINTENANCE-FREE MECHANICALOSCILLATOR;

A SCAN LINE READER HAVING A NONLINEAR SCAN VELOCITY AND A MATCHINGNONLINEAR DATA FLOW RATE.

BRIEF DESCRIPTION OF THE DRAWING

Further objects and advantages of the present reader and the generationof the variable clock will become apparent to those skilled in the artfrom the following detailed description taken in conjunction with thedrawings in which:

FIG. 1 is a block diagram of reader 100;

FIG. 2 is a representation of a typical alpha-numeric character 200 readby scanner reader 100;

FIG. 3 illustrates trace path 310 of beam 124 showing the nonlinearityintroduced by the sinusoidal motion of mechanical oscillator 160;

FIG. 4 shows a block diagram of write rate controller 158 of FIG. 1which compensates for the spacing nonlinearities of FIG. 3; and

FIG. 5 is a timing diagram of the control signals for controller 158.

THE SCANNER-READER SYSTEM

FIG. 1 shows scanner-reader 100 which scans image 104 on a record mediumsuch as document 108 to provide DATA OUT to a data bank 110. A lightsource such as laser 120 provides a light beam 124 which is scannedacross document 108 by a fast horizontal or line scanner device 130 incooperation with slower vertical or page scanning device 134. An opticaldevice 140 columnates and focuses beam 124 into a clear, small readingspot which is scanned across image 104. The light from the reading spotis reflected or absorbed by image 104 and document 108. The reflectedportion is intensity-modulated in accordance with image 104. Lightsensing devices such as photomultipliers 148 detect the reflected lightand provide video analog signals representative of image 104. The videosignals are combined by amplifier 150 and digitized by A/D device 154 toform digital DATA OUT. DATA OUT has a nonlinear bit density due tononlinearities in the horizontal scan velocity of beam 124. A nonlinearvariable clock signal (VAR CLK) generated by write rate controller 158accompanies DATA OUT. The nonlinearities of VAR CLK match the nonlinearbit density of DATA OUT, and DATA OUT is clocked into data bank 110properly on a bit-per-clock basis.

The nonlinearity of the scanning velocity originates in line scanner130, which is preferably a mechanical oscillator 160 of the torsion bartype with electromagnetic drive and pickup, having a stable naturalresonant frequency. Mechanical oscillator 160 drives a pivotinglight-directing device such as mirror 162 which is mounted on thetorsion bar and reflects the focused beam 124 across a deflection angle164 (in dash) causing the writing dot to scan back and forth in the Xdirection across document 108 along trade line 166 (in dash). Theangular displacement of mirror 162 varies sinusoidally due to theresonant nature of mechanical oscillator 160. Mirror 162 has a slowerangular velocity towards the end portion of displacement 164 and afaster angular velocity during the middle portion. These nonlinearitiesproduce corresponding nonlinearities in the scanning velocity. Scan line168 is the most linear central portion of trace 166 and is the mostsuitable for reading.

Page scanning device 134 is a light-deflecting device such as mirror 172mounted on the armature of a galvo motor 174. A linearly increasingvoltage from ramp generator 176 causes galvo motor 174 to rotate slowlyand linearly. Mirror 172 tilts slowly moving beam 124 along the Ydirection of document 108. The minimum ramp voltage defines top margin178T and the maximum ramp voltage defines bottom margin 178B. Scan line168 moves slowly down document 108 defining a left-hand reading margin178L and a right-hand reading margin 178R.

IMAGE QUALITY

FIG. 2 shows a typical alpha-numeric symbol 200 divided into squareelements 210 or bits by the horizontal and vertical scanning of devices130 and 134, and the clocking by VAR CLK. Each bit of DATA OUTcorresponds to one element 210. Symbol 200 is reformed when elements 210are recombined from data bank 110 through an appropriate scannerprinting device. The conventional letter size of four points in the14×10 element format of FIG. 2 requires 2176 data bits in each 81/2 inchscan line, about 3.9 mils per bit. Seven such scan lines are required todigitize a single line of alpha-numeric symbols. Mirror 162 oscillatesat 400 cps making one complete back and forth scan in 2.500milliseconds. Only the most linear 43% of trace line 166 is actuallyemployed in scan line 188 for reading. The actual reading time requiredfor a single 81/4 inch scan in one direction is about 544 microseconds.Therefore, in the FIG. 2 instance, each of the 2177 bits must be readwithin 0.250 microseconds in a 3.9×3.9 mil space with a deviation errorundetectable by the unaided eye. The reading samples of scanner reader100 taken from memory bank 110 were surprisingly crisp and linear inview of these stringent requirements.

The crispness of the image produced by writing from data bank 110 isdetermined by the mechanical precision of reader 100 and by thedefinition of the reading spot. Optics 140 may provide columnatingapertures for eliminating spurious or fringe light. Fuzzy edges aroundthe reading spot may also be reduced by providing a mirror 162 which issmaller in area than the cross-section of beam 124. This mirror clipsthe beam just prior to reading, eliminating the peripheral light whichis the primary source of edge noise and resolution loss. Reducing themirror size has the added advantage of increasing the oscillationfrequency because of the lower angular inertia. A concave clippingmirror may provide the desired focus as well as columnation to reducethe complexity of optics 140. The mirror may be twin-mounted on thedouble torsion bar geometry shown in FIG. 1 which provide counteractivetorques for reducing resonant vibrations throughout mechanicaloscillator 160. In order to reduce swaying, the torsion bar may besecured at both ends as shown in FIG. 4.

VARIABLE CLOCK TO MATCH SCANNING VELOCITY VARIATIONS

FIG. 3 shows the sine trace path 310 of scanning beam on across document108. The Y scan motion in FIG. 3 has been exaggerated relative to the Xscan motion to emphasize the sinusoidal nature of the beam trade line.The horizontal and vertical positions of beam 124 are given by

horizontal position= X= A sine KB

vertical position= Y= (Y scan velocity) (time)

where A is the amplitude of deflection, B is one-half of theinstantaneous deflection angle 164, and K is a normalizing constant=90°/Bmax.

The horizontal component of the velocity of beam 124 is

horizontal velocity= X' = d(A sine KB)dt= A cosine KB.

The horizontal velocity (bold line 320) is not constant and readsuniformly spaced data at a nonlinear rate (shown by dots 330). Dots 330correspond to the position of beam 124 after equal time periods andillustrates the nonlinear nature of the X scan motion. The readingbegins at the left-hand margin 178L of document 108 and terminates atthe right-hand margin 178R. The function of write rate controller 158 isto provide a variable clock (VAR CLK) which adjusts the flow rate ofDATA OUT to match the nonlinear horizontal scan velocity of beam 124.VAR CLK increases in frequency as beam 124 increases in velocity fromleft-hand margin 178L to center scan 360, and VAR CLK decreases infrequency as beam 124 decreases in velocity between center scan 360 andright-hand margin 178R. VAR CLK frequency (the velocity of DATA OUT)closely matches the periodic variations in the scan velocity so thatsamples of DATA OUT printed from data bank 110 do not have visiblenonlinearities in the picture element distribution along the scan lines.

Many possibilities exist for providing a variable clock to synchronizethe clock accompanying DATA OUT with the X scanning velocity. A cosinecontrol signal could be provided to control the clock rate of controller158 in analog fashion. Alternatively, the digital technique described inFIGS. 4 and 5 may be employed. Multiple clock rates are generated andselectively forwarded to data bank 110 during different segments of thesinusoidal scan cycle. The slowest clock is applied during the first orleft-hand segment of scan line 168. Subsequent clocks increase infrequency to match the increase in scan velocity, and increase induration of application because scan line 168 becomes progressively morelinear as the writing dot approaches center line 360. The clocks arethen applied in reverse order for the right-hand side of scan line 168concluding with the lowest frequency clock at right-hand margin 178R.

In the FIG. 4 embodiment scan line 168 is divided into eight segmentsrequiring four separate clock frequencies as follows:

    ______________________________________                                                 Start  Stop     Number                                                        Clock  Clock    of Bits   Frequency                                  ______________________________________                                        Clock I    0        64        64     3.000 mc                                 Clock II   65       320      256     3.270 mc                                 Clock III  321      576      256     3.600 mc                                 Clock IV   577      1088     512     4.000 mc                                 Clock IV   1089     1600     512     --                                       Clock III  1601     1856     256     --                                       Clock II   1857     2112     256     --                                       Clock I    2113     2176      64     --                                       ______________________________________                                    

In applications requiring finer control, more segments and clockfrequencies may be employed.

WRITE RATE CONTROLLER

FIG. 4 shows write rate controller 158 and mechanical oscillator 160 inmore detail. A SYSTEM START pulse from an operator or external controlstarts system control 410 and ramp generator 176. System control 410provides control signals to various other blocks of write ratecontroller 158 for coordinating the operation thereof (see FIG. 5).Mechanical oscillator 160 continually provides a SYNC pulse at X=0 eachcycle to system control 410. System control 410 responds to SYNC byissuing START DELAY. START DELAY activates delay circuit 420 which,after a predetermined delay period, in turn activates a bit counter 430by a START LINE voltage. The delay period permits the reading spot tomove from X=0 to left-hand margin 166L, the start of scan line 156. Bitcounter also identifies the beginning and ending of each of the eightscan line segments and determines the four clock frequencies by means ofFREQ CONT I-IV applied to variable divider 440 which divides 36 mc by 9,10, 11, or 12 in response to FREQ CONT I-IV to obtain VAR CLK I-IV whichadvances bit counter 430.

READER ON is issued from system control 410 to bit counter 430 after topmargin 178T has been established. READER ON causes bit counter 430 toprovide END OF LINE to line counter 450 at the end of each scan line156. Line counter 450 counts one page of scan lines--1408-- andgenerates END OF PAGE which stops system control 410, and indicates thebeginning of bottom margin 178B to the external operator or controlcomputer. READER ON also forwards VAR CLK to A/D 154. clock oscillator490 provides mc signal and multiples thereof for timing the operation ofcontroller 158.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Data bank 110 may be a high-speed memory system in which a solid statememory buffer interfaces with magnetic storage.

Laser 120 may be a 5-20 milliwatt argon laser, Model 162, from SpectraPhysics. Light from laser 120 has twin wavelengths at green (488.0 nm)and blue (514.5 nm) which increases the reading fidelity.

Optics 140 may be a beam expander type optical system from Oriel Co.,Model No. B- 33-60, for focusing laser beam 124 onto document 108, withan aperture for defining the diameter of the reading spot.

Photomultiplier tubes 148 may be any photomultiplier responsive to thetwo wavelengths of laser 120 such as RCA-4523.

Video amplifier 150 may be any wide band video amplifier such as LM 318from National.

A/D 154 may be a

(a) voltage threshold detector formed by an LM 306 from National forsquaring the video analog signal from photomultipliers 148; and

(b) coincidence device such a D flip flop (7474) for establishing a timebase for DATA OUT.

Ramp galvo motor 174 may be any suitable rotating device such as OpticalScanner, Model #G330 from General Scanning.

Ramp generator 176 may be any suitable generator which can provide alinear ramp from 0-5 dc.

Delay circuit 420 is a 12-bit counter clocked by 18 mc formed by three4-bit counters (SN 9316) in cascade. The delay accommodates the timeelapsing between SYNC (when galvo is at X=0) and START LINE (when thescanning beam is at the left-hand margin 178L of the scan line 168).START DELAY presets a control flip flop (SN 74S74) which starts the12-bit counter, and START LINE resets the control flip flop.

Bit counter 430 may be a 12-bit counter formed by three 4-bit modularcounter (SN 9316) in cascade for counting the 2176 bits of each scanline 168. Bit counter 430 also includes an 8×32 PROM (8223) responsiveto six counts which determine the boundaries of the eight segments(counts 64, 320, 576, 1600, 1856, and 2112). The PROM provides threebinary bits of control frequency data to variable divider 440.

Variable divider 440 may be a programmable counter (9316) and aninverter (7404) connected in feedback configuration which are responsiveto the three bits of control frequency data for dividing a 36 mcclocking signal by 9, 10, 11, and 12 to provide VAR CLK I-IV.

Line counter 450 may be a 12-bit counter formed by three 4-bit counters(SN 9316) in cascade for counting the 2816 scan lines in each data blockor page.

Clock oscillator 490 may be an LC oscillator and frequency divider forproviding squarewaves at 36 mc and 18 mc.

We claim as our invention:
 1. A scanner for reading record data from arecord medium by scanning the record medium with an energy beam toprovide a digital data output flow with a nonlinear bit density,comprising:a mechanical oscillator means having a sinusoidaldisplacement characteristic; energy-directing means responsive to thesinusoidal displacement of the oscillator means for causing the energybeam to scan in a predetermined pattern across the record medium causingthe energy of the beam to become modulated by the record data at amodulation density directly proportional to the scanning velocity of theenergy beam; energy sensor means for detecting the modulated energy forproviding a correspondingly modulated analog signal; analog to digitalmeans responsive to the modulated analog signal for providing acorrespondingly modulated digital data output flow having a flow ratedirectly proportional to the relative scan motion between the energybeam and the record medium; and controller for providing a variableclock to accompany the digital data output flow having a series ofpredetermined constant pulse frequencies for predetermined segments ofthe sinusoidal displacement for approximately matching the clock rate tothe digital data output flow rate.
 2. The scanner of claim 1, whereinthe mechanical oscillator is a resonant device with a natural frequencyof resonance.
 3. The scanner of claim 2, wherein the resonant mechanicaloscillator has an arcuate oscillatory displacement.
 4. The scanner ofclaim 3, wherein the energy beam is a light beam.
 5. The scanner ofclaim 4, wherein the energy-directing means is an optical device.
 6. Thescanner of claim 5, wherein the optical energy-directing device isarcuately displaced in response to the arcuate oscillatory displacementof the resonant mechanical oscillator.
 7. The scanner of claim 6,wherein the resonant mechanical oscillator is a torsion bar device whichsupports the light-directing optical device and oscillates about itslongitudinal axis.
 8. The scanner of claim 7, wherein the torsion baroscillator is electromagnetically activated by a drive coil.
 9. Thescanner of claim 7, wherein the arcuate displacement of torsion baroscillator is electromagnetically monitored by a pickup coil to provideperiodic synchronization signals to the controller.
 10. The scanner ofclaim 6, wherein the resonant mechanical oscillator is formed by twintorsion devices mounted in counter-active relationship.
 11. The scannerof claim 6, wherein the optical energy-directing device is transparentand refracts the incident light beam across the record medium.
 12. Thescanner of claim 6, wherein the optical energy-directing device has areflective surface for reflecting the incident light beam across therecord medium.
 13. The scanner of claim 12, wherein the area of thereflective surface is less than the cross-section of the beam causingthe beam to be clipped to the dimensions of the reflective surface. 14.The scanner of claim 2, wherein the controller provides variable clockrates for substantially matching the data output flow rate to the firstderivative of the sinusoidal displacement.
 15. Apparatus for readinginput data at a nonlinear rate from a laser-responsive image on a recordmedium, and writing the data into a data bank on a bit-per-clock basis,comprising:a laser source means for providing a laser beam; a torsionoscillator for providing a resonant sinusoidal angular motion; a laserbeam deflecting means responsive to the sinusoidal angular motion forcausing the modulated laser beam to scan the record medium periodicallyalong a scan line during the most linear portion of the sinusoidalangular motion when the velocity of the scanning laser beam is subjectto the least amount of variation; drive means for establishing relativemotion between the laser beam and the record medium generally traverselyto the scan line for permitting the laser beam to scan the image whichcauses the energy of the laser beam to become modulated by the inputdata therein; photo-sensing devices for detecting the modulated energyand provide an input data modulated video analog signal; a thresholddetector for digitizing the video analog signal forming an outputdigital flow with a bit density which varies as the velocity of thescanning laser beam; and a clock means responsive to the torsionoscillator for providing a variable clock formed by a series ofpredetermined constant clock rates each generated during a predeterminedsegment of the sinusoidal angular motion, and having a frequencyapproximating the bit density of the output digital flow.